1. Field of Invention
This invention generally relates to a process for applying a coating material to a surface of a substrate. More particularly, this invention relates to a process for applying a charge generating material to a photoreceptor substrate, and to photoreceptors made by such a process.
2. Description of Related Art
Among the many conventional methods of coating a substrate with a coating material is the use of an extrusion or slot die from which the coating material is extruded onto the substrate. Using such slot coating of thin layers, the window of operating parameters is extremely small and is affected by factors such as the coating thickness, the speed of the substrate, the theological properties of the coating liquids, the vacuum pressure, the relative speed of the extruded coating material, the amount of pressure applied to the coating material as it progresses through the extrusion slot, etc.
Extrusion coating methods for forming thin layers are described in U.S. Pat. Nos. 4,521,457 and 5,614,260, the entire disclosures of which are totally incorporated herein by reference.
Such extrusion coating methods have conventionally been used to manufacture Xerographic photoreceptors. Xerographic photoreceptors are typically prepared using either a single layer configuration or a multilayer configuration. The multilayer arrangement is more common. In the multilayer configuration, the active layers are the charge generation layer (CGL) and the charge transport layer (CTL). Charge generation layers are usually prepared as dispersions of pigment particles in a polymer host. Most charge generation layers conventionally range from between 0.1 and 5 microns in dry thickness. In contrast, transport layers conventionally range from about 20 to 29 microns thick. In the multilayer configurement, additional layers, such as blocking, adhesion, overcoat and undercoard layers may optionally be included as desired.
Generally, each of the charge generation and charge transport layers is applied separately onto a substrate. The charge generation layer is typically coated onto a blocking layer, under which there can be an undercoat layer for providing adhesion and optionally a blocking function over the substrate. Then, the charge transport layer is typically coated over the charge generation layer.
The use of conventional extrusion slot die methods of forming thin coatings of dispersions of photoconductive particles can produce defects resembling brush marks along each edge of the deposited coating. These brush marks can remain as defects in the dried coating and can ultimately print out as undesirable artifacts in the final electrophotographic copy.
The coating materials for charge generation layers of photoreceptors can be Newtonian but are often made of Non-Newtonian dispersions, which show shear thinning, thixotropic and yield stress behaviors. The dispersion shows little or no deformation up to the yield stress, which can lead to flocculation of dispersion particles in the coated film.
U.S. Pat. No. 5,531,872 to Forgit et al., the entire disclosure of which is incorporated herein by reference, discloses a static process for fabricating a photoconductive member including depositing a photoconductive material, such as a charge generating material, and a charge transport material on a substrate, sequentially in any order, or simultaneously. The photoconductive material, the charge transport material, or both, are electrophoretically deposited onto the substrate from a liquid composition using a voltage of from 8 to 60 volts to create an electric field. The electrophoretic deposition is accomplished by maintaining the electric field for up to five minutes.